On Ajax-enabled forms with a pager, pager is disappearing on form validation error and on Ajax form reload

The test-only patch attached shows the issue.

Related, #1181370: Pager, tablesort links in a form corrupt when reloaded via AJAX (e.g., via exposed filter))

Testbot failure?

OK, the test fails as expected.

My understanding of why this happens: on validation error and on Ajax reload the form is reloaded from the cache, and buildForm is not executed. This leads to the pager theme to be rendered without the underlying $pager_* globals initialised, hence no pager rendered.

I have no idea on how to get around this though, nor whether there are other patches in the pipeline addressing this.

4: pager_form-2182555-1-test-only.patch queued for re-testing.

Just checked whether #1959574: Remove the deprecated Drupal 7 Ajax API made any difference. It didn't.

Well, I do not know if it's the way to go... This patch:

1. Stores the state of the $pager_* global variables in a $form_state['pagers_state'] entry when caching the form and if at least one pager is initialised.
2. Restores the state of $pager_* global variables from the $form_state['pagers_state'] cache entry when getting the form from cache and the entry exists.

Note: the pager links are built with the wrong href (the path to the Ajax controller) when Ajax-reloading the form. I did not touch that here, there's already #1181370: Pager, tablesort links in a form corrupt when reloaded via AJAX (e.g., via exposed filter)) for that.

Looks like #11 broke PHPunit tests. Here I moved the pack/unpack functions from pager.inc to protected methods in FormBuilder.

13: pager_form-2182555-13.patch queued for re-testing.

Reroll after #340723: Make modules and installation profiles only require .info.yml files

PSR-4 reroll

Rerolled.

Reroll of #23 after commit of #1588138: pager_query_add_page() [in D7, theme_pager_link()] overrides parameters passed programmatically.

Working on some cleanup.

#1588138: pager_query_add_page() [in D7, theme_pager_link()] overrides parameters passed programmatically and the patch in this issue were both introducing a pager_test module. Now #1588138: pager_query_add_page() [in D7, theme_pager_link()] overrides parameters passed programmatically was committed and the reroll in #27 would contain some duplicated code in the test module. So here I am just cleaning up the pager_test module and the pager tests.

Rerolled

This should fix failures - needed to change '#theme' => 'pager' to '#type' => 'pager'.

#2263569: Bypass form caching by default for forms using #ajax. fixed this issue by removing form caching for AJAX forms, so I am going to mark this issue fixed too.

The test only patch in #2504709-5: Prevent _wrapper_format and ajax_form parameters from bleeding through to generated URLs is an evolution of the test-only patch in #23, and demonstrates how this is no longer happening.